The present invention relates generally to thin film coatings. More specifically, the present invention relates to methods and apparatus for monitoring selected optical characteristics of thin film coatings during the deposition process, and controlling the deposition process responsive thereto.
Optical filters comprising thin films, and particular multilayer films, have many commercial applications. For example, precision optical filters have found widespread use in the fiber optic telecommunications industry in Dense Wavelength Division Multiplexing (“DWDM”). It is well known that the performance of multi-layer thin film coatings can be improved by precisely controlling the thickness of the individual layers forming the multilayer coating.
In the example of DWDM filters, the filters are formed to transmit substantially all of the light within a corresponding wavelength band or channel while reflecting substantially all of the light outside of the channel. It is well known that an optical filter meeting the desired characteristics of transmitting substantially all of the light within a desired wavelength band and reflecting substantially all of the light outside of the band may be formed from a series of quarter wave stacks (“QWS”) with interposed thick cavity layers. QWS filters may be formed by depositing alternating layers of materials having differing indices of refraction. Typically, one material (L) such as silica has a relatively low index of refraction, and the other material (H) such as tantalum pentoxide has a relatively high index of refraction. The QWS is formed by depositing a layer of H material, then a layer of L material, then a layer of H material, and so on until the filter reaches the desired optical characteristics.
When forming multi-layer thin film coatings, the optical performance of the coating can be greatly improved by precisely controlling the thickness of the individual layers. It is possible to mathematically calculate the desired thicknesses of the alternating layers and thus the “cut-off” points for each layer may be determined from the known deposition rate of each material. However, the known methods of depositing thin films are not accurate enough to rely solely on the mathematical calculation to control the deposition process. One known method to overcome the inaccuracies of the deposition process to obtain useful coatings is to control the deposition process as a function of the optical characteristics of the deposited coating.
For example, one known method of making precision optical filters is by forming the thin film coatings in an e-beam evaporation process with the assistance of an ion gun. With reference to FIG. 1, a deposition chamber 10 encloses the point sources 12, 14 of the H and L materials and the substrate 16 to be coated. Typically, the substrate 16 is circular having a diameter of about 3 inches and may be rotated at about 1000 rpm to improve the uniformity of the coating about the surface of the substrate. The materials may be deposited in alternate layers by alternating the activation of the sources of the materials. The thickness of each layer may be controlled by directing a beam of light 18 at the wavelength of interest (λc) from the source 20 and monitoring the transmissivity of the coated substrate 16 at that wavelength λc by measuring the intensity of the light beam 18 at the detector 22. The deposition of each material may be “cut off” when the monitor shows that the thickness of the material being deposited has reached the “cut-off” point. The deposition process in an e-beam evaporation process typically takes about five minutes per layer.
However, as illustrated, such optical monitoring systems are found in systems where the position of the substrate relative to the source is fixed (although the substrate may be rotated about its axis). It is often desirable to form thin film coatings in “batch” processes where a large number of substrates forming an array is translated past the sources of material to be deposited. In such batch processes, it is known to optically monitor a witness substrate that remains stationary. However, due to the spatial variations of the coating flux within the deposition chamber, the array of substrates being coated are exposed to different coating fluxes from the other substrates in the array and the witness substrate and thus the coating is formed at a different rates on different substrates in the array. This leads to coating variations among the array of substrates. Uniformity of coating among the array of substrates may be improved by moving the substrates throughout the chamber, however, the witness substrate remains stationary. There remains a need for methods and apparatus for optically monitoring substrates that translate past the sources of coating material, and for optically monitoring multiple substrates in an array. There further remains a need for optically monitoring substrates wherein the angle of incidence of the monitoring beam changes as the substrate translates past the beam source.
Accordingly, it is an object of the present invention to obviate many of the above deficiencies in the prior art and to provide a novel method and apparatus for monitoring selected optical characteristics of a coating formed on a substrate during the deposition process.
It is another object of the present invention to provide a novel method and apparatus for monitoring selected optical characteristics of coatings formed on substrates translating past the source of coating material.
It is yet another object of the present invention to provide a novel method and apparatus for monitoring selected optical characteristics of coatings formed on multiple substrates in an array.
It is still another object of the present invention to provide a novel method and apparatus for monitoring selected optical characteristics of coatings formed on substrates where the angle of incidence of the monitor beam changes as the substrate translates past the beam source.
It is a further object of the present invention to provide a novel method and apparatus for monitoring selected optical characteristics of coatings formed on substrates in a sputter deposition process.
It is yet a further object of the present invention to provide a novel method and apparatus for monitoring selected optical characteristics of coatings formed on substrates in a sputter deposition process wherein the substrates are carried by a rotating drum.
It is still a further object of the present invention to provide a novel method and apparatus for improving the signal to noise ratio in systems for monitoring selected optical characteristics of coatings formed on substrates.
It is yet a further object of the present invention to provide a novel method and apparatus for aligning the components in systems for monitoring selected optical characteristics of coatings formed on substrates.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.